(1) Field of the Invention
The present invention relates to a method and system of carrier frequency allocation in a digital mobile communication system in order to realize an effective use of the carrier frequency and to improve the receiving reliability of the communication system. More particularly, the invention relates to a method of allocation of mutually shifted carrier frequencies for various radio coverage zones covering a certain service area.
(2) Description of the Prior Art
When a selection call is to be made in a mobile communication system having a service area consisting of a plurality of radio coverage zones, it is necessary to transmit the same signal simultaneously from a plurality of transmitters to cover entire service area. In the conventional method of allocation of the radio frequency channels, either of the following two methods have been used. One is to allocate different frequency radio channels for each of the zones, and the other is to use the same frequency radio channel common to all of the zones.
In the former case, a large number of radio channels are required in order to avoid interference, as a result the efficiency in utilizing frequencies is lowered. Furthermore additional disadvantages are unavoidable, such as the requirement for multi-channel receivers in the mobile vehicles, and the increase of the lost call rate owing to a fact that several seconds of channel switching time is required. On the other hand in the second method a highly accurate carrier synchronizing system should be employed, and the calling reliability is lowered due to interference between the same frequency waves at the boundaries between radio coverage zones.